Method and apparatus for coating a substrate

ABSTRACT

An improved method of coating one or more layers of liquid coating composition onto a substrate from a coating hopper of the type having an inclined slide surface, wherein each coating composition is fed in the form of a layer, and in superposed relation to any other composition layers, upon an upper portion of the inclined slide surface to flow by gravity toward the substrate. According to the invention, the layer(s) is redirected at a lower lip portion of the slide surface to flow in a more horizontal direction before flowing across a generally horizontal gap onto a substrate moving in a generally upward direction.

[ 1 Dec. 23, 1975 METHOD AND APPARATUS FOR COATING A SUBSTRATE [75]Inventor: Brian W. Jackson, Pinner, England [73] Assignee: Eastman KodakCompany,

Rochester, NY.

[22] Filed: Jan. 26, 1973 [21] Appl. No.: 326,621

[56] References Cited UNITED STATES PATENTS 3,413,143 ll/1968 Cameron etal. 427/402 3,526,528 9/1970 Takahachi et a1. 117/81 3,531,314 9/1970Kerr et al 117/34 3,607,345 9/1971 Thomas et al. 117/34 3,627,56412/1971 Mercier 117/34 3,640,752 2/1972 lchiwati et al. 117/34 FOREIGNPATENTS OR APPLICATIONS 1,151,173 7/1963 Germany 117/34 PrimaryExaminerMichael Sofocleous Attorney, Agent, or Firm-A. P. Lorenzo [57]ABSTRACT An improved method of coating one or more layers of liquidcoating composition onto a substrate from a coating hopper of the typehaving an inclined slide surface, wherein each coating composition isfed in the form of a layer, and in superposed relation to any othercomposition layers, upon an upper portion of the inclined slide surfaceto flow by gravity toward the substrate. According to the invention, thelayer(s) is redirected at a lower lip portion of the slide surface toflow in a more horizontal direction before flowing across a generallyhorizontal gap onto a substrate moving in a generally upward direction.

8 Claims, 3 Drawing Figures US. Patent Dec. 23, 1975 METHOD ANDAPPARATUS FOR COATING A SUBSTRATE BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to improvements in coatingmethods and apparatus. The invention is particularly useful in thecoating of one or more layers of coating compositions onto a substrate,such as, say, a web.

'2. Description of the Prior Art In the prior art, as represented by US.Pat. Nos. 2,761,419, filed in the name of Mercier et al. and 2,761,791filed in the name of Russell et al. the contents of which patents areherein incorporated by reference-it is known to simultaneously coat amoving substrate, for example, a web of photographic film base withemulsion coating compositions, by the use of a coating hopper whichincludes one or more inclined slide surfaces, down which one or more ofthe fluid coating compositions may be made to flow. The hopper mayinclude a plurality of separate exit slots, whereby respective liquidcoating compositions may be metered from individual supplies anddistributed uniformly across respective inclined slide surfaces. Each ofthe respective compositions flows by gravity as a layer down itsrespective inclined surface, whereby the layer becomes smooth and ofuniform thickness. The slide surfaces are arranged so that the layersflow on top of one another. At the end of the last slide surface, i.e.,the one adjacent to the substrate, the stratified layers flow into abead or puddle which bridges a small generally horizontal gap between anedge of the coating hopper and the upwardly moving substrate. Thesubstrate, as it is advanced into contact with the bead, simultaneouslypicks up all the layers, which layers deposit on the substrate as acomposite coating of substantially distinct superimposed layers.

While the above apparatus functions satisfactorily, various defects canarise in the coated layers if the bead is disturbed. For example, aparticularly noticeable fault is the appearance of longitudinalstriations which render the coated web unacceptable as a commercialtageous to create a pressure differential between the exposed surfacesof the bead, such as by the creation of a vacuum on the trailing surfaceof the bead to eliminate excessive vibration and/or rupture of the bead.The range of vacuum levels which may be used is, of course, limited as,if it is too great, the high pressure differential across the bead mayalso cause the bead to be disturbed and/or ruptured. At times, it maydevelop that the amount of vacuum being used is insufficient toeliminate the excessive vibration in the bead, and thus it is desirableto increase the amount of such vacuum. Where an increase in vacuum willitself cause disturbance and/or rupture of the bead, it is sometimesnecessary to change coating conditions, such as by lowering web speed,to make satisfactory coatings.

A further problem with the use of the coating hopper described in theaforementioned Mercier et al. patent is that particles in the liquidcoating composition may adhere to the hopper at the lip edge thereof andcause 2 undesirable streaks to be formed in the coating. An apparatusand method which reduces or minimizes the deleterious effects of suchparticles on coatings represents a significant contribution to the stateof the art.

SUMMARY OF THE INVENTION It is therefore one object of the invention toprovide an improved method and apparatus for coating a moving substrateat higher speeds with minimum bead disturbance.

It is a further object of the invention to provide an improved methodand apparatus for coating a moving substrate which minimizes theundesirable effects of particles which adhere to a lip of the coatingapparatus.

In the apparatus and method of the invention, a coating hopper isprovided with a downwardly inclined slide surface upon which one or morelayers of liquid coating composition(s) may be made to flow so thelayers become smooth and of uniform thickness. The hopper includes a lipwhich extends towards the substrate at an obtuse angle relative to theslide surface and the layer(s) of coating composition(s) may flow onthis lip before being fed into a bridge of coating solution,

. which bridge spans a generally horizontal gap between the hopper andan upwardly moving substrate to be coated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partly insection, showing a coating apparatus including a multiple layer slidehopper as known in the prior art; and

FIG. 2 is a close-up side view of an improved coating apparatus made inaccordance with the invention; and

FIG. 3 is a view similar to FIG. 2 showing another embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Inprior art coatingapparatus, such as disclosed in the Mercier et al. patent and shownherein in FIG. 1, a substrate, such as web W, is supported on andadvanced past a coating hopper 10 by a driven coating roller 13. The webmay be paper, metal or plastic film and may include one or moresubstantially dry coatings previously applied thereto. For illustrativepurposes the hopper 10 is shown as being adapted to coat two layerssimultaneously onto the web W although it will be appreciated that slidehoppers are known in the prior art to coat one or more than two layersand the invention is also applicable to such hoppers. Metering pumps, Pand P, are coupled to respective sources of fluid coating compositionsand feed the compositions 16,23 at desired rates into respectivecavities 11,19 formed in the interior of hopper 10. The compositions16,23 are each then forced to flow generally vertically as a ribbonthrough narrow discharge slots 14,21 respectively, each of which 'slotsat one end extends into the interior of the hopper to a respectivecavity 11,19 and at the other end exits onto the upper end of adownwardly inclined planar slide surface 15,22 respectively. Thecompositions 16,23 are each thus extruded in the form of a liquid layeronto their respective slide surface. The slide surfaces 15,22 in theprior art multiple layer hopper shown in FIG. 1 are substantiallycoplanar; however, they may be substantially parallel with slide surface22 raised slightly above slide surface 15 a distance of the order of thethickness of layer 23. The discharge slots 14,21 are each defined by apair of opposed spaced-apart parallel planar surfaces 14',l4"; 21, 21"respectively and are arranged so that an exit of the lowermost slot 14is spaced above an edge 27 of the coating hopper 10. At the exit of thelowermost slot 14, the upper layer 23 flows upon the lower layer 16,whereupon the two layers flow together along the surface 15 instratified relationship into a bead at 17. The bead 17 spans agenerallyhorizontal gap between the edge 27 and the upwardly moving webW. As used herein a web or substrate is generally upwardly moving at anypoint where at such point its vertical component of velocity is greaterin magnitude than its horizontal component of velocity.

As the web is advanced rapidly past the bead, a surface thereof picks upthe layers and the layers deposit onto the substrate as a compositecoating of substantially distinct superimposed layers. To stabilize thebead, suction or a vacuum may be employed on the trailing surface 24 ofthe head to establish a pressure differential between the exposedsurfaces of the bead. The suction may be provided by a chamber 25 whichis coupled to a vacuum pump 26 to exhaust air from the chamber 25.

In FIG. 2, a portion of an improved slide hopper 29 made in accordancewith the present invention is shown. For purposes of clarity the suctionchamber has been deleted from FIGS. 2 and 3 but such pressuredifferential establishing means is advantageous when used in conjunctionwith the embodiments shown in FIGS. 2 and 3. The slide hopper 29 issimilar in most respects to the hopper shown in FIG. 1, but differs inthat it includes an upturned lip 31 which extends at an obtuse anglefrom the lower end of the lowermost slide surface 32 toward the surfaceof a generally upwardly moving substrate W, e.g., the web, being coated.The lip 31 is preferably generally planar and of the same transversewidth as the planar slide 32 and terminates in a transverse edge 34,defined by the planar land of the lip 31 and side face 39 of the hopper.Edge 34 is spaced generally horizontally from the coating roller 33 adistance slightly greater than the thickness of the web W, to be coatedso as to establish in operation of the hopper a bead or puddle 37 ofliquid coating compositions. Preferably the gap between edge 34 and thesurface of the web being coated is between about 0.005 inches (0.13 mm)to about 0.03 inches (076mm). As may be noted from FIG. 2, the coatingsolutions flow down the lowermost slide surface 32 with the uppermostlayer 35 on top of the lowermost layer 36. The slide surfaces 30,32 areeach inclined relative to the horizontal at a desired angle that ispreferably greater than ten degrees and less than 45 depending upon theproperties of the composition(s) being coated. As the lip 31 is oflesser inclination to the horizontal than that of the slide surfaces30,32 99 the layers 35,36 flow over the lip with reduced speed butincreased thickness (depth). The increased thickness of the layers onthe upturned lip 31 is advantageous in that the effects of any particlesthat happen to lodge at edge 34 are minimized by a dilution of sucheffects by the thicker layers on the upturned lip, i.e., a particle of acertain size that is lodged on the edge 34 will tend to create greaterdisturbances in thin (shallow) layers as opposed to thick (deep) layersas the liquid compositions flow past the particle. The lip 31 isdirected in a generally horizontal direction, and is preferablyhorizontal although it may be inclined relative to the horizontal in thedirection of fluid flow within the preferred range of :5 degrees. Thelength of the lip 31 in the direction of liquid flow is advantageouslyin the range of about 0.025 to 0.080 inches (0.06cm to 0.2lcm), and ispreferably about 0.040 inches (010cm). Such dimensions are advantageousin the coating of photographic compositions wherein emulsion coatingsmight vary between a dry thickness of 0.0001 0.001 inches and protectivelayers separating such emulsion layers might be as thin as one micron.

With the use of the coating roller 33 to support the web in a smoothcondition it is desirable to have the point of application of the beadto the web W be' within a preferred (acute) angular range of applicationpoints and such preferred range is from about a point on the web that islocated at about 40 below the horizontal radius of the coating roller 33vertically to about 30 above said horizontal radius. The generallyhorizontal gap is advantageous in that it allows gravity to act on thesuspending bridge of coating solution and facilitates the formation ofthe bead.

In the prior art apparatus of FIG. 1, one factor which may contribute tothe inability of the bead at times to withstand higher pressuredifferentials is that the bead must, in addition to the pressuredifferential across it, support itself against the force of thedownwardly rushing liquid layers which are continuously feeding into thehead.

The introduction of the coating compositions into the bead along thegenerally horizontal plane of the lip 31 apparently is able to reducethe force of the liquid rushing into the bead or otherwise increase thestability of the bead so that increased suction levels may be used onthe bead at high coating speeds without encountering bead disturbancewhich produces unsatisfactory coatings. Furthermore, even at the samesuction levels, increased coating speeds may be obtained using theapparatus and method of the present invention over that shown in theMercier et al. patent.

To illustrate the advantages of the invention two superimposed layers ofgelatin solution may be coated using a coating hopper of the prior artas shown in FIG. 1 and embodying the invention as shown in FIG. 2. Thelower gelatin solution may have a concentration of 3.85% by weight and awet laydown of 1.125 lbs/ square feet. The upper layer may have aconcentration of 6.6% by weight and a wet laydown of 0.303 lbs/100square feet. Suction may be maintained across the head during coatingwith a gap between the lip and the web of about 0.008 inch. With theapparatus of the present invention it was found that striations due toinstability of the bead and which are present with the use of beadcoating apparatus of the prior art are absent at high coating speeds.

If desired, the transverse edge 34 may be rounded or chamfered toprevent nicks or other damage to the otherwise sharp edge. A coatinghopper with such a chamfered edge 34" is shown in FIG. 3 wherein likenumerals refer to parts similar to that shown and described for FIG. 2.Preferably the lip 31 in the embodiment shown in FIG. 3 should be of thesame dimension and angular orientation as for that described for FIG. 2.

A coating hopper which includes such a chamfered edge 34" isadvantageous in that with the use of such a hopper there issubstantially eliminated the formation of streaks in coating, whichstreaks may be attributed to imperfections in a sharp transverse lipedge.

While the preferred embodiment has been described with respect tomultiple layer coating apparatus, the

invention may also be used to advantage in the form of a single layerslide hopper wherein a single layer of liquid coating composition may bemade to flow down an inclined slide surface, and then along an upturnedlip before being introduced into a bead, which bead bridges a generallyhorizontal gap between the hopper and a generally upwardly moving web.In such a single layer slide hopper the preferred degree of inclinationof the slide surface and that of the lip and the preferred length of thelip are within the ranges referred to for the double layer slide hopper.To demonstrate the advantages of the above-described apparatus of theinvention, a layer of a 5.55 percent by weight aqueous solution ofgelatin was formed on a slide surface, and was coated onto a web with awet laydown of 1.8 lbs/I00 square feet. A differential air pressure wasmaintained across the coating bead as the layer passed over the edge ofthe lip of the slide surface. The gap between the lip and the moving webwas about 0.005 inch. As compared with results obtained under similarcoating conditions using a planar slide surface of the prior art whichdid not have an upturned lip, it was found that, with apparatus of thepresent invention, longitudinal striations due to instability of thebead are absent or minimized at coating speeds at whichthey would occurwith the prior art apparatus. Thus acceptable coatings could be made atspeeds substantially higher than that using the planar slide hoppers of;the prior art.

In its broader aspects, the invention may be embodied as a slide hopperhaving a curved slide surface(s), such as of the type shown in FIG. 3 ofU.S. Pat. No. 2,681,294. However, coating hoppers havingplanar slidesurfaces are preferred, as they are substantially easier and lessexpensive to machine. In machining the planar land 31 on the lowermostplanar slide surface 32, it is desirable to round, as shown in FIG. 2,the obtuse angle 38 formed between the surfaces 31,32.

The invention is particularly applicable to a multiple layer slidehopper of the extrusion-slide type, as disclosed in U.S. Pat. No.2,761,417, which may also have a generally upturned lip formed on theslide surface. Such a coating hopper is described and claimed incopending commonly assigned US. application Ser. No. 327,048simultaneously filed herewith in the name of Jackson et al. and underthe title Method and Apparatus For Coating a Multiple Number of Layersonto a Substrate.

The method and apparatus of this invention can be utilized to coat anymaterial or mixture of materials which can be put in liquid form andbead coated, for example, in the form of a solution, a dispersion, or asuspension. In many instances where this method finds application, thecoating composition is an aqueous composition but other liquid vehiclesof either an organic or inorganic nature, can also be utilized and arefully within the contemplation of this invention. The respective layerscan be formed of the same or different liquid coating compositions andthese coating compositions can be either miscible or immiscible with oneAs indicated hereinbefore, the method and apparatus of this inventionare especially useful in the photographic art for manufacture ofmultilayer photographic elements, i.e., elements comprised of a supportcoated with a plurality of superimposed layers of photographic coatingcomposition. The number of individual layers may range from two to asmany as ten or more. In the photographic art, the liquid coatingcompositionsutilized are of relatively low viscosity, i.e., viscositiesfrom as low as about 2 centipoise to as high as about 150 centipoise, orsomewhat higher, and most commonly in the range from about 5 to about100 centipoise.

The method and apparatus of this invention are suitable for use with anyliquid photographic coating composition and can be employed with anytype of photographic support and it is, accordingly, intended to includeall such coating compositions and supports as are utilized in thephotographic art within the scope of these terms, as employed herein andin the appended claims.

The term photographic normally refers to a radiation sensitive material,but not all of the layers presently applied to a support in themanufacture of photographic elements are, in themselves, radiationsensitive.

For example, subbing layers, pelloid protective layers, filter layers,antihalation layers, etc., are often applied separately and/or incombination and these particular layers are not radiation sensitive. Thepresent invention relates also to the application of such layers, andthe term photographic coating composition, as employed herein, isintended to include the compositions from which such layers are formed.Moreover, the invention includes within its scope all radiationsensitive materials, including electrophotographic materials andmaterials sensitive to invisible radiation as well as those sensitive tovisible radiation. While, as mentioned hereinbefore, the layers aregenerally coated from aqueous media, the invention is not so limitedsince other liquid vehicles are known in the manufacture of photographicelements and the invention is also applicable to and useful in coatingfrom such liquid vehicles.

More specifically, the photographic layers coated according to themethod of this invention can contain light-sensitive materials, such assilver halides, zinc oxide, titanium dioxide, diazonium salts,light-sensitive dyes, etc., as well as other ingredients known to theart for use in photographic layers, for example, chemical sensitizers,development modifiers, antifoggants and stabilizers, developing agents,hardeners, plasticizers and lubricants, coating aids, matting agents,antistatic agents, brighteners, spectral sensitizers, absorbing andfilter dyes, color materials, etc. Specific examples of theseingredients may be found in the publication entitled Product LicensingIndex, Volume 92, December 1971, Publication 9232, pages 107 through 110, and in the publications cited therein. The photographic layers canalso contain various colloids either alone or in combination asvehicles. Suitable hydrophilic vehicle materials include bothnaturally-occurring substances, such as proteins, for example, gelatin,gelatin derivatives, etc., and synthetic polymeric substances, such aswater soluble polyvinyl compounds. Further examples of suitable vehiclesare disclosed in the aforecited publication.

Various types of supports may be used to support the photographicelements. Typical flexible supports include film base, e.g., celluloseacetate and poly(ethylene terephthalate), paper, metal, etc. Othersupports which may be used are also disclosed in the aforecitedpublication.

It will be appreciated from the above that the invention provides animproved apparatus and method which may be used to coat a web atincreased speeds. It will be further appreciated that such apparatusembodying the invention claimed herein may be made relativelyinexpensively by modifying coating hoppers of the prior art.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

I claim:

1. A method of coating a liquid coating composition onto a web substratefrom a coating hopper having a downwardly inclined slide, which methodcomprises the steps of:

feeding the liquid coating composition in the form of a layer upon anupperportion of the inclined slide surface such that the liquid flows asa layer by gravity downwardly on the inclined slide surface toward thesubstrate;

altering the direction of flow of the layer at a lower lip portion ofthe slide surface to a direction closer to horizontal than the directionof the upper portion of the slide surface so that the liquid compositionflows more horizontally as it approaches the substrate;

flowing the liquid composition from the lower lip of the slide surfaceacross a generally horizontal gap to the substrate; and

moving the substrate .in a generally upward direction across and incontact with the coating composition so that the surface of thesubstrate is coated with the coating composition.

2. The invention according to claim 1 wherein the coating compositionflows along said lower lip portion for a distance of at least about0.025 inches.

3. The invention according to claim 2 wherein the coating compositionflows along said lower lip portion in a direction which is in the rangeof about i-5 relative to the horizontal.

4. The invention according to claim 3 wherein the coating composition isan aqueous hydrophilic colloidal solution.

5. The invention according to claim 4 wherein the coating compositionbridging said gap forms a bead.

6. The invention according to claim 5 wherein a pressure differential isestablished between the exposed surfaces of the bead so that arelatively lower pressure is created adjacent the trailing surface ofthe bead.

7. The invention according to claim 3 wherein the liquid coatingcomposition is a photographic coating composition.

8. A method of simultaneously coating a plurality of distinct superposedlayers of liquid coating compositions onto a web substrate from acoating hopper of the type having a downwardly inclined slide surface,which method comprises the steps of:

simultaneously feeding each liquid coating composition in the form of alayer and in superposed relation to the other composition layers ontothe upper portion of the slide surface inclined downwardly at a firstpredetermined angle with respect to horizontal such that the liquidcoating composition layers flow down the surface by gravity;

altering the direction of flow of the layers at a lower portion of theslide surface to a second predetermined angle of inclination less thansaid first angle of inclination so that the liquid composition layersflow more horizontally;

flowing the liquid layers from the lower portion of the slide surfaceacross a generally horizontal gap to a substrate to be coated; and

moving the substrate in a generally upward direction across and incontact with the liquid composition layers so that the surface of thesubstrate engages the lowermost layer and simultaneouslypicks up all ofsaid compositions in distinct superposed layers.

1. A method of coating a liquid coating composition onto a web substratefrom a coating hopper having a downwardly inclined slide, which methodcomprises the steps of: feeding the liquid coating composition in theform of a layer upon an upper portion of the inclined slide surface suchthat the liquid flows as a layer by gravity downwardly on the inclinedslide surface toward the substrate; altering the direction of flow ofthe layer at a lower lip portion of the slide surface to a directioncloser to horizontal than the direction of the upper portion of theslide surface so that the liquid composition flows more horizontally asit approaches the substrate; flowing the liquid composition from thelower Lip of the slide surface across a generally horizontal gap to thesubstrate; and moving the substrate in a generally upward directionacross and in contact with the coating composition so that the surfaceof the substrate is coated with the coating composition.
 2. Theinvention according to claim 1 wherein the coating composition flowsalong said lower lip portion for a distance of at least about 0.025inches.
 3. The invention according to claim 2 wherein the coatingcomposition flows along said lower lip portion in a direction which isin the range of about + or - 5* relative to the horizontal.
 4. Theinvention according to claim 3 wherein the coating composition is anaqueous hydrophilic colloidal solution.
 5. The invention according toclaim 4 wherein the coating composition bridging said gap forms a bead.6. The invention according to claim 5 wherein a pressure differential isestablished between the exposed surfaces of the bead so that arelatively lower pressure is created adjacent the trailing surface ofthe bead.
 7. The invention according to claim 3 wherein the liquidcoating composition is a photographic coating composition.
 8. A METHODOF SIMULTANEOUSLY COATING A PLURALITY OF DISTINCT SUPERPOSED LAYERS OFLIQUID COATING COMPOSITIONS ONTO A WEB SUBSTRATE FROM A COATING HOPPEROF THE TYPE HAVING A DOWNWARDLY INCLINED SLIDE SURFACE, WHICH METHODCOMPRISES THE STEPS OF: SIMULTANEOUSLY FEEDING EACH LIQUID COATINGCOMPOSITION IN THE FORM OF A LAYER AND IN SUPERPOSED RELATION TO THEOTHER COMPOSITION LAYERS ONTO THE UPPER PORTION OF THE SLIDE SURFACEINCLINED DOWNWARDLY AT A FIRST PREDETERMINED ANGLE WITH RESPECT TOHORIZONTAL SUCH THAT THE LIQUID COATING COMPOSITION LAYERS FLOW DOWN THESURFACE BY GRAVITY; ALTERING THE DIRECTION OF FLOW OF THE LAYERS AT ALOWER PORTION OF THE SLIDE SURFACE TO A SECOND PREDETERMINED ANGLE OFINCLINATION LESS THAN SAID FIRST ANGLE OF INCLINATION SO THAT THE LIQUIDCOMPOSITION LAYERS FLOW MORE HORIZONTALLY; FLOWING THE LIQUID LAYERSFROM THE LOWER PORTION OF THE SLIDE SURFACE ACROSS A GENERALLYHORIZONTAL GAP TO A SUBSTRATE TO BE COATED; AND MOVING THE SUBSTRATE INA GENERALLY UPWARD DIRECTION ACROSS AND IN CONTACT WITH THE LIQUIDCOMPOSITION LAYERS SO THAT THE SURFACE OF THE SUBSTRATE ENGAGES THELOWERMOST LAYER AND SIMULTANEOUSKY PICKS UP ALL OF SAID COMPOSITIONS INDISTRICT SUPERPOSED LAYERS.